This invention relates to pressure-sensitive membrane-type touch panel devices which electrically indicate the X-Y coordinates of contact of an operator's finger or probe or which may be used as switches.
Membrane touch panels of the general type with which the invention is concerned are well known in the art. These touch panels generally include a pair of circuit layers, one fixed and one moveable, separated by spacer means. The circuit layers comprise sheets of insulating material which have conductive circuit patterns provided thereon. In some analog applications, the circuit patterns may comprise continuous layers of conductive material provided over the surfaces of the respective circuit layers. In any case, these circuit patterns face each other and are separated by spacer means formed of insulative material, which may comprise insulative dot spacers, a separate insulative layer in which apertures are formed, a grid pattern formed of insulative material, and the like. Electrical switching is effected by applying finger or other pressure to specific locations of one of the circuit sheets to move a contact element, i.e. a region of the circuit pattern, on that circuit sheet through a void of the spacer means, e.g., through the region between adjacent dot spacers or through an aperture of an apertured spacer layer, to make contact with a contact element on the other circuit sheet. Touch panels of this type are generally formed in a laminate construction with the layers bonded together. The panel may also include one or more of an overlay sheet with graphics or the like, a layer adapted to be electrically charged during operation to provide EMI shielding, and a backer plate to support the touch panel. The backer plate may comprise a CRT screen in which case the layers of the touch panel are made of transparent material.
Membrane-type touch panels of the type described above may find applications in conditions of varying temperature and humidity. Extended use under such environmental conditions, or even advancing age of the touch panel itself, may cause circuit layers to become warped to a greater or lesser degree. This becomes a problem when the warping is so great that a circuit element on the warped circuit layer becomes situated in a void of the spacer means and contacts a circuit element on the other circuit layer in which case a false actuation of the panel occurs. Attempts have been made to overcome this problem by modifying the size of the voids of the spacer means. For example, in the case where the spacer means comprise dot spacers, the distance between the dot spacers has ben reduced in an attempt to prevent false actuations. However, any reduction in the distance between the dot spacers results in a corresponding increase in the force required for the operator to actuate the touch panel switch. Since maximum allowable actuation forces are usually specified, this method of preventing false actuations has to the present not been practical in all cases. Indeed, the distance between the dot spacers may be reduced to the point where actuation of the touch panel becomes impossible.
It is also seen from the foregoing that the force required to actuate a membrane touch switch, as noted above, depends to some extent on the size of the voids of the spacer means which maintain the separation of the opposed circuit patterns. For example, in touch panels where the spacer means comprise an insulative apertured layer provided between the circuit layers, the diameter of the apertures directly affects the magnitude of the force required for actuation. Similarly, in touch panels where the spacer means comprise dot spacers, the distance between the dots has a direct affect upon the magnitude of the actuation force. Increasing the diameter of the apertures of a spacer layer or the distance between dot spacers, i.e., increasing the lateral dimensions of the voids of the spacer means, on the one hand, decreases the required actuation force, but, on the other hand, increases the possibility of false actuations by making it easier for opposed regions of the facing circuit patterns to inadvertently contact each other.